Before dropping the tank, I’d make absolutely sure the DEF quality fault is actually being caused by the DEF head. On these trucks, wiring/connectors, the quality sensor and contamination can all trigger the same type of fault.
If you’ve already confirmed the head needs replacing, I’d recommend draining the tank completely and cleaning it out while you have it down. Inspect the DEF lines and connectors carefully for crystallization or corrosion, and use only the correct DEF afterward.
Having the rear supported properly and plenty of room underneath will make the job much easier. Take plenty of pictures of the line routing and connector locations before disconnecting everything.
Hopefully someone with a 2018 DS FL chassis can confirm whether the tank actually needs to come completely out or if there’s enough clearance to access the head without dropping the whole assembly.
